This invention is related to the recovery of oil from a hydrocarbon-bearing subterranean formation by injection of a water-soluble polymer in an aqueous solution into the oil reservoir to stimulate additional production and recovery of hydrocarbons. More specifically, the polymer employed is a modified heteropolysaccharide produced by the bacterium Xanthobacter sp.
Generally, oil or hydrocarbons are recovered from subterranean formations BY initially employing primary recovery techniques. Once primary production is no longer economically feasible, some form of enhanced recovery is applied to these formations to abstract further quantities of oil. One of the earliest and most popular forms of enhanced oil recovery is water injection in which water or brine is injected into the hydrocarbon containing formation to force the residual hydrocarbons contained therein through the formation to a production well which is placed at an appropriate location. Since the viscosity of the hydrocarbons present in the hydrocarbon bearing formation is usually higher than the viscosity of water or other fluids injected into the formation, the quantity of hydrocarbons removed by such methods is small and further, frequently results in the bypassing of a substantial portion of the hydrocarbons by the less viscous water. This effect is referred to as viscous fingering. This situation is further aggravated by the presence of zones of high permeabilityat various levels in the hydrocarbon bearing formation. These so-called "thief" zones also permit the escape of a substantial portion of the relatively low viscosity water or brine without any displacement of hydrocarbons.
Other problems have arisen in the use of water-soluble bacterial polysaccharides for enhanced oil recovery since the viscous injection fluid (2-2000 cps) is often a diluted fermentation broth (0.01-1.0% w/v) which may contain residual cell bodies, digested solid nutrients and polymer aggregates (microgels). For most polysaccharides, aggregation is exacerbated by the elevated concentrations of mono-, di- and trivalent cations found in reservoir brines which may form specific ionic complexes between polymer chains and/or may generally reduce the "solvent power" of the aqueous media allowing the polymer to readily self-associate. The operational result of aggregation is to reduce the injection rate and increase the injection pressure required to introduce the polymer solution into the subterranean oil reservoir. Further, in some extreme cases, polymer molecules aggregated into networks may cause plugging in the porous reservoir rock matrix which could ultimately result in permanent damage to the formation. In the present invention, the ability to inject a heteropolysaccharide into a hydrocarbon-containing reservoir is significantly improved by including the compound beta-fluoropyruvate in the fermentation medium of the organism Xanthobacter sp. This effect is especially pronounced at elevated salinity.
Thus, it is an object of the present invention to provide an improved process for recovering hydrocarbons from subterranean hydrocarbon bearing formation.
Also, it is an object of the present invention to provide an improved additive, i.e., drive fluid, for enhancing the recovery of such hydrocarbons.